The overall goal of the proposed research is to determine the pathway(s) by which visual information is transmitted to the superior colliculus (SC) neurons that innervate nuclei which, in turn, control head and eye movements. The sensory information that deep layer, SC cells send to medullary and spinal structures is thought to be critically involved in orienting and attentional behavior. However, the visual input pathways for these SC neurons have not been defined. Previous physiological and light microscopic experiments have suggested several potential sources of critical visual input to deep layer output cells in the rodent SC. Superficially directed dendrites of deep layer neurons may receive retinal input and/or synapses from the axon collaterals of visually responsive superficial layer cells. Alternatively, direct retinal projections to the deep laminae and/or axonal projections from the superficial to the deep layers might provide the necessary visual input for these cells. We will combine anterograde transport, intracellular horseradish peroxidase injection, and electron microscopic techniques to determine which of these potential pathways actually exist in the hamster's SC. We will then combine electrophysiological recording with reversible inactivation techniques to determine which of the pathways defined in the electron microscope provide(s) the necessary visual input to deep layer output cells.